Braiding is a cost effective process for producing cylindrical structures for advanced composites. For example, carbon fibers have been braided into the shapes of helicopter drive shafts, launch tubes for shoulder-launched assault weapons, and even rocket nozzles. When the fibers are subsequently resin impregnated and cured, the resultant parts are lightweight and exhibit high strengths. In addition, composites have a high resistance to most chemical and environmental threats.
Braiding flat structures is also known. Although, there are other method of fabricating composite parts including hand lay-up of layers of fabric, braiding, is a preferred method in many instances since braiding is a highly effective process in terms of material usage. This is primarily due to the fact that the plies of a structure are created in-process, without wastage. No cutting or kitting is required. And, dry "biased" fibers, i.e., those orientated at an angle, may be braided on presently available braiding machines. Biased fibers in a flat structure are desirable because they are able to handle shear loads.
In contrast, fabrication processes which begin with tape or woven products must include the step of cutting bias plies from unbiased sections causing waste. Thus, braiding provides the dual advantage of using composites in their least cost material form and doing so in a material efficient manner.
Besides being an automated process, braiding is extremely attractive because of its versatility. A standard tubular braid can be used to fabricate any mandrel configuration which can be passed through it. Braiding is excellent for producing non-symmetrical structures with complex curvatures. Finally, standard braiders can be modified to produce a wide range of components.
One problem with all composite parts, however, is that they are difficult to join together. Most attachments schemes center around those acceptable for metals, i.e. using fasteners such as rivets, bolts, clips and the like. See U.S. Pat. No. 4,109,435, especially FIG. 4.
Fastening two composite parts is especially troublesome when one continuous structure intersects another continuous structure, i.e. where stringer members intersect a frame member, where rib members intersect bar members, or where longerons intersect with bulkheads. An example is an aircraft fuselage which is made up of a thin skin supported by frame members and intersecting stringers.
Often, to attach composite stringer members to a frame member, a "mouse hole" is cut in the frame to allow the stringer to pass through. Fasteners and additional lay up of fabric plies then secure the stringer/frame intersection. This joining process adds weight, labor, cost, and results in possible stress and fatigue points.
If the fibers of the plies of the stringer members are all non-biased, (i.e., parallel and running in the direction of and perpendicular to the stringer) and the fibers and the frame are similarly non-biased, weaving schemes to form a continuous intersection at the joint of the stringer and frame might not be too troublesome.
Constructions which include only these non-biased fibers, however, are not very strong in shear. Angled or "biased" fibers, on the other hand, in both the frame and the intersecting stringers, although exhibiting high shear strength, make it very difficult to weave a continuous intersection. So, frame members and stringer members are usually fabricated separately and then joined by cutting a hole in the frame member as described above.